1. Field of the Invention
The present invention relates to an image forming apparatus for converting character or figure information into bit mapped information (pixel information) and forming an image based on the converted information.
Also, the present invention relates to an image recording apparatus such as a laser printer, for example, which has a spare paper feeding function.
In this specification, the term "spare paper feeding function" means a function of starting to feed a spare sheet of recording paper prior to reception of a signal indicating the start of recording.
2. Related Background Art
Electrophotographic printers are able to produce prints with high resolution and high quality. Therefore, various types of electrophotographic printers such as laser printers, LED printers and liquid crystal printers have recently been developed and become widespread. By utilizing a high quality feature, those printers are used to output complex figures and pictures.
A controller (e.g., a postscript controller) for processing complex image data covering one page of printed sheet requires an image memory (hereinafter referred to as a page memory) with a capacity corresponding to one page at a minimum. To print a sheet of A4 size with resolution of 300 dpi, for example, a page memory having a capacity as large as 1 Mbyte is required.
There is a large amount of image information to be processed by such high quality printers. Accordingly, image data to be handled by memory devices of computers and other systems for image processing equipment are often in the form of coded data or programmed data rather than raw raster image data.
The performance of page printers is determined by how fast coded image information of one page is converted into raster image information of one page before printing.
One example of control form in page printers of a prior art is as follows.
FIG. 1 shows a section of a laser beam printer as typical a one of page printer. FIG. 2 is a schematic block diagram of a control circuit in the typical laser beam printer. In FIG. 2, reference numeral 25 represents an information processing apparatus (such as a personal computer or work station, for example) external to the laser printer. Reference numeral 27 denotes an external interface (such as Centronics or RS232C, for example) which delivers coded image information (such as ASCII code, for example, hereinafter referred to as coded information) to the laser printer. The coded information is received by an interface circuit 18 in the laser printer. A microprocessor 17 receives the coded information, received by the interface circuit 18, through an internal bus 28. The internal bus 28 comprises a collection of a data bus, an address bus and a control bus. The microprocessor 17 is operated in accordance with a control program stored in a memory 21. The memory 21 is a non-volatile ROM.
The microprocessor 17 processes the coded information obtained through the interface circuit 18 to some extent and stores it in a memory 19. Thus, the memory 19 is a RAM for storing the coded information. The microprocessor 17 sequentially stores the coded information externally received into the RAM 19 and, at the same time, converts the coded information into image information representing a dot image, followed by storing it in a RAM 20. Thus, the RAM 20 is an image data store memory (bit map memory). Denoted by 22 is a DMA controller serving to read out the data stored in the RAM 20 and deliver the read data to a raster conversion circuit 24.
The DMA controller 22 may exclusively use the internal bus 28 independently of the microprocessor 17. When it is detected that the image data stored in the RAM 20 has reached an amount corresponding to one page (i.e., all the coded data of one page have been converted into the image data), the microprocessor 17 sets the DMA controller 22 in an active state. The DMA controller 22 exclusively uses the internal bus alternately with the microprocessor 17. Upon a request from the raster conversion circuit 24, the DMA controller 32 sequentially reads the image data out of the RAM 20 and delivers the read data to the raster conversion circuit 24. The raster conversion circuit 24 converts the parallel image data received from the DMA controller 22 into serial image data. Then, in synchronism with a horizontal synch signal, the serial image data are outputted to a laser driver (not shown) in a mechanical control unit 26 for modulating a laser beam.
Mechanical control in the laser printer will be described below with reference to FIG. 1. In FIG. 1, reference numeral 1 represents a body of the laser printer. After developing the coded data of one page and storing the image data in the memory 20, the microprocessor 17 rotates a feed motor (not shown) through an I/O driver 23. This starts rotating a photosensitive drum 2, a primary charge roller 5, a development roller 7, a transfer roller 10, a fixer roller 15A, and a discharge roller 16. The feed motor is controlled in its rotation by the mechanical control unit 26.
Reference numeral 3 represents a laser scanner which houses a laser scan mirror, a laser scan motor, a laser emitting element, and a laser drive circuit therein. The I/O driver 23 serves to rotate not only the feed motor but also the laser scan motor within the laser scanner 3. The I/O driver 23 also sequentially applies high voltage biases to the primary charge roller 5, the development roller 7 and the transfer roller 10. Further, the I/O driver 23 turns on a clutch mounted on a paper feed roller 12 to feed sheets of transfer material 13 such as paper stacked in a paper cassette 14 one by one. The sheet of transfer material 13 thus fed is once stopped by a resist roller 11. Then, the mechanical control unit 26 informs the I/O driver 23 that the fed sheet of transfer material 13 has reached the resist roller 11. At the time the sheet of transfer material 13 is stopped by the resist roller 11, the microprocessor 17 sets the DMA controller 22 in an active state. Subsequently, the serial image data are delivered from the raster conversion circuit 24. The delivered serial image data are inputted to the laser scanner 3 so that a laser beam modulated by the image data irradiates the photosensitive drum 2. This builds a latent image on the surface of a photoreceptor, the latent image being visualized into a toner image by a development unit 6.
The sheet of transfer material 13 once stopped by the resist roller 11 starts to be fed again by the resist roller 11, and the toner image is transferred to the sheet of transfer material 13 by the transfer roller 10. The sheet of transfer material 13 with toner deposited thereon is fixed under heating by the fixer roller 15A and, thereafter, it is discharged by the discharge roller 16 externally of the printer body. The remaining toner that has not been transferred to the sheet of transfer material 13 by the transfer roller 10 is collected by a cleaner 9.
While the fixer roller 15A is generally adjusted to a predetermined temperature, i.e., a temperature for printing (during the fixing operation), it may be also adjusted to a lower setting temperature, i.e., a standby temperature, other than that printing temperature. This aims to prevent a temperature rise in the printer, reduce electric power consumption, etc.
In this way, the coded information given from the external information processing apparatus is printed as image information on the sheet of paper.
When printing data in an amount corresponding to plural pages, the printing is performed in accordance with the timed relationship shown in FIG. 3. Referring to FIG. 3, the microprocessor 17 starts reception of the coded information at the timing (a). Simultaneously, the microprocessor 17 starts the image development and stores the image data in the memory 20. After terminating the reception of the coded data for the first page at the timing (b), the microprocessor 17 successively starts reception of the coded information at the timing (c). If image development for the first page is completed at the timing (d), the feed motor is rotated at the timing (f) to perform the paper feeding operation. Then, the resist roller 11 is driven at the timing (g) and reading of the image data by the DMA controller 22 is started at the timing (h). Simultaneously, the serial image data are produced by the raster conversion circuit 24 and laser exposure is started at the timing (h). Afterward, the laser exposure for the first page is completed at the timing (i). Since the reception of the coded information for the second page has already been completed at the timing (e), the image development for the second page is also started at the timing (i). After that, the second page is printed following the same sequence as the first page.
As will be seen from FIG. 3, the periods of (a) to (d) and (i) to (j) of the image development are completely independent of the period of (h) to (i) for reading the image data (also the period for the laser exposure), with no overlapping periods therebetween. This is because the image memory has a capacity corresponding to only one page.
With the foregoing control method, no access is made to the image memory 20 during the period of (f) to (h) (or the period of (k) to (m). Accordingly, the throughput (the number of sheets printed per unit time) is lowered in laser beam printers of the type that the distance between the paper feed roller 12 and the resist roller 11 is very long. Providing the image memory with a capacity corresponding to two pages makes it possible to overlap the period of the image development and the period of reading the image data with each other, and thus increase the throughput. In this case, however, the memory cost is doubled.
To solve the aforementioned disadvantage, the following control method could be envisaged.
With this method, at the time the microprocessor 17 receives the coded information of one page from the external information processing apparatus 25 such as a host computer, the feeding operation for the sheet of transfer material 13 such as paper is started and then stopped at a predetermined position in a standby state. Afterward, at the time the microprocessor 17 has completely finished the development of the coded information into the image data representing the dot image, the image data representing the dot image are sequentially transferred as the serial image data to the mechanical control unit 26, so that the laser beam is modulated to expose the surface of the electrophotographic photosensitive drum 2. At the same time, in synchronism with the exposure image, the sheet of transfer material 13 starts to be fed again from the standby state.
However, the above control method raises a problem discussed below when practiced while implementing usual temperature adjustment in two modes; i.e., a standby temperature and a printing temperature.
When the process shifts into the printing operation after completion of the image development, a certain period of time is required for the fixer roller 15A to reach the printing temperature.
For example, assuming that the fixer roller 15a used comprises an aluminum-made core with a wall thickness of 2 mm, assuming that the roller's outer diameter is .phi. 25 and the roller's length is 260 mm and the roller has a fluoroplastic layer of 30 .mu.m or thereabout coated on the core, assuming that a halogen heater is used in the fixer roller 15A having output power of 400 W, and assuming that the standby temperature is 165.degree. C., and the printing temperature is 180.degree. C., it takes about 6 seconds for the fixer roller 15A to heat from the standby temperature to the printing temperature.
Naturally, the above period of time is prolonged as the weight of the roller is increased, the heater output power is reduced, and the difference between the standby temperature and the printing temperature becomes large.
If the temperature adjustment mode of the fixer roller 15A is shifted to a lower temperature for the standby state to wait for the end of the image development as mentioned above, the period of time required to be prepared for the next printing operation would be dependent on the period of time required for the fixer roller 15A to reach a state capable of starting the fixing operation. This results in the problem of lowering throughput in terms of the sheets of transfer material printed per unit time.
Another laser beam printer of the prior art is arranged as shown in FIG. 4. Referring to FIG. 4, denoted by 201 is a photoreceptor drum as a carrier for an electrostatic latent image, 202 denotes a charge roller for uniformly charging the photoreceptor drum, 207 denotes a scanner motor for scanning a laser beam 206 over the photoreceptor drum 201, 203 denotes a developer for developing the electrostatic latent image, created by a laser beam 206 on the photoreceptor drum 201, using toner, 204 denotes a transfer roller for transferring a toner image to a sheet of print paper, 205 denotes a cleaner for removing the non-transferred toner remaining the photoreceptor drum 201, 208 denotes a fixer for fixing the toner on the sheet of print paper thereto, 209 is a paper feed roller, 211 denotes a pre-resist sensor, and 210 denotes a resist roller for synchronizing the paper feeding and the image production.
A print control system of the printer thus arranged is shown in FIG. 3.
Denoted by 300 is a printer controller for developing an image code signal from a host computer or the like into a dot signal, outputting a print request signal and a spare paper feed signal to a printer engine control unit 301, and further delivering the developed dot data. 301 is a printer engine control unit for controlling communication with the printer controller 300 and various components of the engine. A ROM of the printer engine control unit stores a control program shown as a flow chart of FIG. 6. 302 denotes a fixer control unit for turning on and off a fixer heater based on a control signal from the printer engine control unit 301. 303 denotes a scanner control unit for controlling energization and de-energization of the scanner motor based on a control signal from the printer engine control unit 301. 304 denotes a high voltage control unit for controlling a timed sequence of the charging, development and transfer based on a control signal from the printer engine control unit 301. 305 denotes a paper feed control unit for controlling the spare paper feeding to stop the sheet of print paper after passing the resist sensor, and the paper feeding to be performed for the normal printing, based on a control signal from the printer engine control unit 301. 306 denotes a control line for transferring the communication between the printer controller and the printer engine control unit, the image data, the print request signal, the spare feed signal, etc. 307 to 310 denotes control lines for transferring data between the printer engine control unit and the other various units.
A flow chart representing a sequence of spare paper feeding and subsequent paper feeding is shown in FIG. 6. The printer engine control unit checks whether or not it is in a print signal receivable state (S201). If the print signal receivable state is determined, then the printer engine control unit waits for a spare paper feed signal while waiting for a print signal (S202, S203). If the print signal is received, then the spare paper feed operation is started and, at the same time, a paper feed jamming detection timer is started (S212-S214). Then, the printer engine control unit checks whether or not the resist sensor detects a sheet of paper (S215), while it also checks whether or not the paper feed jamming detection timer is timed up in its counting (S224). If the sheet of paper has not reached the resist sensor before the time-up, then the control flow goes to a jamming treatment (S225). If the sheet of paper has reached the resist sensor before the time-up, then the printer engine control unit waits for a period of time To sufficient for the sheet of paper to strike against the resist roller and make a loop in the predetermined amount (S216). Subsequently, it stops the paper feed roller and outputs a vertical synch request signal (S216-1, S217), followed by waiting for reception of a vertical synch signal (S218). Thereafter, upon receiving the vertical synch signal, the resist roller is driven to start its rotation and the image is written for completing a printing operation (S219, S220).
If the spare paper feed signal is received prior to the reception of the print signal (S203), then the paper feed roller is driven to start its rotation (S204) and, at the same time, the paper feed jamming detection timer is started (S205). Then, the printer engine control unit checks whether or not the sheet of paper has reached the resist sensor (S206), while it also checks whether or not the paper feed jamming detection timer is timed up in its counting (S207). If the timer has been timed up before the sheet of paper reaches the resist sensor, then the control flow goes to a jamming treatment (S208). If the sheet of paper has reached the resist sensor before the time-up (S206), the paper feed roller is stopped at that moment (S209), and a spare paper feed end flag is set (S210), followed by coming into a print signal standby state (S211).
If the print signal is received during that print signal standby state (S202), then the spare paper feed end flag is reset (S221). Subsequently, the printer engine control unit drives the paper feed roller to start its rotation (S222), and waits for a period of time T1 sufficient for the sheet of paper to strike against the resist roller and make a loop in the predetermined amount after passing the resist sensor (S223). Thereafter, similarly to the above normal printing, operation the printer engine control unit outputs a vertical synch request signal, waits for reception of the vertical synch signal and, upon receiving the vertical synch signal, drives the resist roller to start the printing operation.
Further, if the print request signal is not received until the elapse of a predetermined time after one page has been completely printed, the printer engine control unit performs a post-rotation sequence. FIG. 7 shows a timing chart of primary signals generated during the process of printing of one sheet in a normal manner, printing another sheet with the spare paper feeding, and coming to the stop after the post-rotation sequence. Conventionally, in the post-rotation sequence, the charge, development and transfer are turned off in this order, followed by stopping the scanner motor and the main motor. That sequence is controlled in the same manner even when the sheet of paper having been preliminarily fed remains unprinted.
However, the prior art has suffered from the following disadvantages because the post-rotation sequence is performed in the same manner as the normal printing even with the preliminarily fed sheet of paper being in the standby state.
(1) Even when the spare paper feed signal is received in the standby state where the feed motor and the scanner motor are stopped while waiting for the print request signal and the spare paper feed signal, the sheet of paper is just fed into a predetermined position. Thus, upon receiving the print signal, the sheet of paper is fed again after waiting for the scanner to reach the predetermined number of revolutions. Accordingly, the control unit fails to sufficiently fulfill the function of the spare paper feed signal delivered for the purpose of speeding up the first printing.
(2) Delivery of the spare paper feed signal from the control unit implies the fact that the control unit will perform printing in near future. But, since the post-rotation sequence is effected to stop the scanner upon the elapse of the predetermined time after the spare paper feeding, it is disabled to feed the sheet of paper until the scanner has reached the predetermined number of revolutions, even with the print signal received subsequently. This remarkably impairs the advantage of the spare paper feeding.
(3) In addition, the fixer temperature is switched to the standby temperature fairly lower than the printing temperature after the stop of the main motor. Therefore, if the post-rotation sequence is once ended, the paper feeding must be started upon the subsequent print signal after waiting for the fixer temperature to reach the printing temperature, in spite of the preceding spare paper feeding.